Keep it a secret
by IzzyLightwood9
Summary: What happens when Jace and Isabelle are left alone to spar and no one's around? A sudden attraction pulls them together during their training and almost pushes them over the edge. One-shot. JacexIsabelle. Post COG.


**A/N: Hello my fellow fan-fictioners :) So I'm back! I decided to write a small one-shot about Jace and Isabelle and I'm pretty proud of it. I know it's nothing like my long story 'Unforgettable' but its pretty good :) I hope you like it! I like pairing random characters together and this is just an idea that popped into my head the other day.**

**Enjoy!**

**Izzy.**

**Disclaimer: I do not own anything from The Mortal Instruments. Just the story plot.**

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"Izzy! Hurry up! We aren't going to wait forever you know!" Isabelle heard Alec scream from somewhere down the hall of the Institute.

Isabelle had just gotten home from shopping with Clary and her room was littered from clothes with prices tags on them and empty shopping bags. Shopping was Isabelle's favorite thing in the world, well besides kicking demon ass and stringing along cute guys. Clary had agreed to go shopping with Isabelle for some new training clothes since her old ones were not up to her current fashion standards. Not any old t-shirt and shorts would do for Isabelle. No, she had to also be fashionable while training with Jace and Alec.

Isabelle was just about to head off and train with Jace and Alec but first, she had to find something to wear. Isabelle surveyed her cluttered floor once, before rummaging through a pile of clothes on her bed. Shirts were flying and the colors of shirts seemed to blur together as she tried to hurry and get ready, she was already late as it was. A pair of skin tight, black, yoga pants fell off the bed and landed on her feet. She looked down skeptically and picked up the pants observing them all over. They were a new pair she had just gotten today and they needed to be broken in some time. Isabelle shrugged and decided that they would have to do.

She quickly slipped off her black, leather, skinny jeans and her purple stilettos that laced around her feet. She pulled on the yoga pants and rolled the waistband down once, to where her hip bones were just barley jutting out of the sides. She looked around her room once again in search of her brand new tank top she had just bought also.

She glanced over her shoulder and spotted it lying by her vanity mirror and quickly strode over to throw it on. After she had the tank top on she turned towards her mirror to look herself over for any flaws. The white tank top she had on was also skin tight, like her pants, and hugged her curves in all the right places. She also wore a black tank top underneath it that lined the outside of her white one, to make sure you couldn't see through it if she was to sweat. It was pulled up a little to where an inch or two of her skin was showing between the hem of her shirt and the waist line of her pants. Her makeup was still applied from the shopping spree and she decided to leave it on since Alec would yell at her if she was any second later than she had to be. She threw her hair up into a ponytail, with a few loose strands hanging around her face lightly. She looked herself up and down one last time before turning around and making her way out the door.

"Isabeeeeeeeeeelle!" Alec called from the training room.

"I'm coming, I'm coming! Don't get your panties in a wad Alec!" Isabelle responded back and giggled when she heard Alec groan.

Isabelle finally stepped through the doorway of the training room and immediately averted her eyes to Alec and his tapping foot. Isabelle laughed and shook her head back and forth.

"I swear you look more like mom than I do." Isabelle giggled and placed her hands on her hips, mocking Alec's stance.

"Oh, HA HA! You're so funny Isabelle!" Alec drawled out sarcastically. "And you're an hour late! Where the hell have you been? You missed weapons training and target practice."

"I know Alec, I know. Clary and I got caught up and time got away from us. Oh, and by the way, she told me to tell you that she's skipping training today because she had to go help Jocelyn with something but she said she'd be back later." Isabelle said in an exasperated breath.

Alec glared at Isabelle for a moment before rolling his eyes and uncrossing his arms from his chest. "Alright, whatever, looks like it's just you and Jace then."

"Wait what do you mean just me and Jace? Where are you going?" Isabelle asked.

Alec gave a little smirk and looked down at his bland, grey cell phone. He looked back up at Isabelle and slid the phone into his pocket. "Well unlike you sister dearest I got my training done before anything else. I'm done for the day. I'm going to Magnus's and you're going to stay here with Jace. The least you can do, if you do anything at all, is spar with Jace some. He wanted to practice some new moves but we never got around to it."

Alec turned on his heel and made his way towards the doors of the training room. When he got outside the doors he looked over his shoulder and called back out to Isabelle. "Have fun!" Isabelle heard him giggle as he slammed the doors shut and Isabelle was left in silence.

In the distance Isabelle heard the punching bag being rung off its hinges. Jace must be throwing a few punches before their sparring session. While Jace was getting out the last of his blows Isabelle took this free opportunity to stretch out a bit. She walk to the center of the mat and turned around to where her back was facing the doors that lead to the weight room. Isabelle pulled her right arm across her chest and held it there for ten seconds then alternated arms. She bounced up and down on her toes and took a couple of deep breaths before she felt her top half starting to loosen up. She rolled her neck around on her shoulders a few times, loving the feeling it sent down her spine. She closed her eyes and released a deep breath.

She figured that was enough for her upper half since Jace was probably tired from training and wouldn't be too much of a challenge for her. Isabelle stood on one foot and grabbed her left ankle pulling it behind her to stretch put her thigh muscles. She did this for ten seconds before switching over to the right leg. Once she was done with her thighs she decided on stretching out her hamstrings. Those were the muscles she usually pulled while sparring. Isabelle arched her back before bending over and grabbing her left ankle. Her legs were shoulder-width apart and her ass was stuck up in the air, like it always was whenever she stretched out her hamstrings. Isabelle released her breath when she felt the pulling and aching sensation that coursed through her leg. To her it was and amazing feeling, loosening up her muscles.

Once she felt like she had stretched out her left leg good enough she switched over to her right leg, just like all her other stretches. When she began to feel the same sensation she let her eyes fall shut and her head hang limp. Isabelle was still in the middle of her stretch when she heard the doors to the weight room open and somebody's breath catch. Isabelle looked between her legs to see a sweaty and shirtless Jace standing their wide eyed and lips pursed ever so slightly.

"Like what you see Herondale?" Isabelle smirked and gave Jace a wink.

Jace shook his head trying to get rid of his expression before he returned a smirk of his own. "No, I was just noticing how tight those pants are."

Isabelle stood up and turned to face Jace. She raised an eyebrow and placed her hands on her hips. "Yeah, and you we're totally staring at my ass."

Jace let out a low chuckle and shook his head. "Maybe I was. Maybe I wasn't. You can't prove anything, Lightwood."

Isabelle rolled her eyes and looked down at her pants. "I have to admit they are pretty sexy. You know you love them." Isabelle said before turning her focus to the center of them mat.

Jace walked around and took a fighting stance across the mat from Isabelle.

"Maybe a little too much." Jace winked and started walking around the outer edge of the mat and Isabelle did the same.

They were parallel to each other as they both continued making their way around the mat in a circle.

"Alec said you've got some new moves. Ha, you're gonna need them if you think you can take me down." Isabelle mocked Jace.

"Alright Izzy," Jace said. "But don't go crying to Alec when I kick your ass."

"Bring it on pretty boy." Isabelle snapped.

Jace's senses snapped and he lunged across the mat towards Isabelle. Isabelle easy blocked Jace's first blow with her left forearm and she quickly regained her fighting stance. Jace lashed out with his left arm and barley nicked Isabelle on the shoulder before she ducked and rolled across the mat and was now standing directly behind him. She threw a few jabs to his back and ducked again before his outstretched arm could connect with her face. Jace threw a quick kick to Isabelle's hip as she tried to regain her stance again. Relentless amounts of kicks and punches were being thrown back and forth between the two, most of them were blocked but every once in a while someone would get a good lick from the other.

Jace was growing tired and he could tell Isabelle was too, he needed to end it. He just needed to wait for the perfect moment to make his final move. A few more punches and kicks were thrown before Jace saw the perfect opening. Isabelle lashed out with her leg a little too far and Jace quickly grabbed her ankle. He reached out and grabbed her hip, picked her up off the ground and slammed her down onto the mat. He quickly crawled on top of her and pinned her arms and legs down with his own rather forcefully.

"I can't take you down huh? Ha, yeah right." Jace laughed mere inches from her face.

Jace felt Isabelle wiggle underneath him and he tightened his grip.

"Jace let me up!" Isabelle gasped.

"No, not until you admit that you were wrong." Jace said.

"Please Jace," Isabelle wheezed. "You're hurting me."

Jace looked down and saw the pain behind her eyes. He felt bad for being so rough with her and he instantly loosened his grip. He let go of her arms and leaned up some.

"Izzy, I'm so sorry… I didn't mean to-" Jace's words were cut off when Isabelle blinked and had a mischievous look in her eye.

In a split second, Isabelle wrapped her leg around the back of Jace's knee and firmly grasped his forearms. In one swift motion she rolled and flipped Jace onto his back to where she was now on top of him. She quickly straddled his hips and pinned his wrists down beside his head. She leaned down to where she was mere centimeters from his face.

"Now…" Isabelle purred sensually. "What were you saying?"

Jace felt Isabelle's hot breath flow over his cheek and down the side of his neck. This made him shiver and heat flood throughout his body. Jace looked up into Isabelle's smirking face and devious eyes.

"Cheater." Jace spat and tried to wriggle out of Isabelle's grasp but she had a firm hold.

"How so?" Isabelle hissed.

Isabelle's actions were clouding Jace's thoughts and causing a growing sensation in his pants. He had never been played like this by Isabelle before, he had always thought of her as a sister. But the thing was, she wasn't his sister. There was no relation at all, and boy, he never realized how freakin' sexy she was.

"You're uh- you uhm…" Jace stuttered. "You're pants are distracting!" He blurted out.

Isabelle looked down at him skeptically before turning her mouth up into a sly grin. "You think that's distracting," Isabelle joked. "Then you're going to think this is torture."

Jace didn't know what she was talking about until, suddenly; he felt Isabelle's hips being grounded into his. Jace let out a sharp moan and his eyes rolled back into his head. She ran a long manicured fingernail down the center crease in his abs and caused him to arch off the ground even further.

"Ha, I think I won Herondale." Isabelle laughed as she let go of Jace's wrists and started to get off of him.

Before she could even start to stand up, she felt a hand fly out and grab her wrist. She felt herself being pulled back down on top of Jace.

"And just where do you think your going Izzy? You aren't getting off the hook that easily." Jace growled.

"Jace we can't-" Jace put a finger to Isabelle's lips and cut off her sentence.

"Izzy," Jace pleaded. "Can we just let go? For once don't think."

Isabelle looked down and couldn't help but get lost in Jace's golden, amber eyes. She had never realized before how breathtaking, and absolutely gorgeous he was. She never thought about him that way but being alone with him, in this moment, she couldn't help but think of anything else.

"Fuck, Jace…" Isabelle gasped and tilted her head down to where their lips were centimeters apart. She wasn't going to make the first move. No, she would let Jace come to her. She wanted this so badly but she had to know if Jace did too.

Isabelle felt once of Jace's arms lace around her lower back and pull her down on top of him. She let out a muffled moan and tried keep most of her weight off of him by her hands that were planted on either side of his head but their hips and groins were pressed together rather forcefully.

Jace tilted his head up towards Isabelle's and captured his lips with hers. The kiss was gentle and their lips slowly caressed each others. Jace's hands managed to snake around and grip the sides of Isabelle's hips viciously. Isabelle thought that his grip was going to leave bruises but she didn't care. Sometimes she liked it rough.

She shifted her hips into a better position and crashed her mouth back down onto Jace's, harder and more forceful this time. Jace made no intention of pulling away, instead he returned Isabelle's kiss with the same amount of fire and passion that she was giving him.

Jace felt Isabelle's tongue slowly run along the bottom of his lip asking for entrance. Jace willingly opened his mouth and let the girl inside to play. He instantly connected Isabelle's tongue with his as they explored each others mouths.

Isabelle was rocking back and forth against Jace, and grinding her hips into his own, which made low, deep moans erupt from his throat. He could feel his member hardening dangerously.

Obviously Isabelle caught on to the growing erection in Jace's pants, and slowly slid her hands down and cupped the bulge in his jeans. Jace moaned and couldn't help but wrap his arms around her, cup her ass, and pull her down harder against him. He needed the connection and the friction.

Isabelle tore away from Jace's mouth and slowly began trailing kisses down the side of his neck and chest. Jace arched his back off the mat and tilted his head back. He felt Isabelle taking small nips against his chest and rolling the tuffs of skin gently between her teeth. Jace gritted his teeth and groaned Isabelle's name over and over again. She continued trailing her kisses across his chest but slowly slipped her fingers inside the waistband of Jace's shorts. She tilted her head up again and captured Jace's mouth in another heated kiss with her own. She played with the hem of his boxers before slowly dipping her hands further into Jace's pants and under his boxers.

She felt Jace tense underneath her and another muffled moan vibrated his body. Isabelle trailed her fingers across a tuff of blonde hair close to Jace's erection. She slowly played and teased each strand with affection and desire, and gently tugged on it every once in a while.

Jace's hips bucked off the floor and Isabelle smiled into the kiss, knowing and loving the fact that she could cause his body to act like this. Jace leaned up until he was sitting straight up with Isabelle still straddling his lap, their lips and tongues never losing contact.

Isabelle pulled her hands away from Jace's pants and gently laced them behind his neck, pulling him against her to deepen the touch and their kiss. Jace grabbed the hem of Isabelle's tank top and gently tugged it up her body. Isabelle didn't reject and simply just raised her arms for Jace to pull it off. As soon as Isabelle's top was discarded onto the floor Jace's eyes instantly averted to Isabelle's black, lacey bra. Jace gave Isabelle a small smirk before connecting his lips to hers again.

Jace moved his hands from Isabelle's waist and slowly started trailing them up towards her chest, causing her to shiver with anticipation. Isabelle's hands were dug deep into Jace's golden curls and tugging lightly on the ends. Jace's hands finally made their way up to Isabelle's chest and gently cupped her breasts, causing a low moan to come from the girl. Jace loved how he could be so straight forward with Isabelle, with Clary he always had to be cautious and watch what he did, but with Isabelle, it was nothing like that at all.

Jace was just about to unclasp Isabelle's bra when he heard the Institutes elevator coming up to the main floor. Jace placed one last firm kiss against Isabelle's mouth before pulling away and looking up into her sky blue eyes. Both of them were breathless but small smiles graced their faces.

"Awh," Jace whined. "Looks like the fun's over."

Isabelle looked at him before playfully slapping him on the shoulder. "This never happened. Agreed?"

Jace nodded in agreement knowing that this couldn't get out.

Isabelle got up off of Jace's lap and went to put her shirt back on. Jace stood up and made his way over to Isabelle just as the shirt slipped over her head.

"Shall we go see who had the guts to interrupt us?" Jace laughed playfully.

Isabelle smiled and nodded her head. "But like I said Jace, this never happened."

Jace shook his head. "Whatever you say Izzy, we can keep it a secret." Jace laughed as she turned around and gave her a swift smack on the ass.

Isabelle whirled around and glared daggers at Jace before lightening up and giving him a small smile. They both walked in silence to the kitchen to see who was their. Once they entered the doorway they were met by the voice of a gleaming red head.

"Hey guys!" Clary yipped and walked over to Jace.

Isabelle made her way to the sink to get a quick sip of water, and then turned around to face the couple.

"Have a good workout? You guys look pretty flustered and sweaty. Sorry I wasn't there today, I hope you managed okay." Clary added in.

Clary had her back to Isabelle which gave Isabelle a quick opportunity to give a small wink at Jace. Jace saw Isabelle's gesture and gave her a small smirk across Clary's shoulder.

"Oh we managed perfectly fine toady." Jace laughed.

Clary smiled and gave Jace a big hug. Jace returned the hug but couldn't help but avert his eyes to a certain black haired vixen, standing in the back of the room.

Isabelle saw Jace looking at her and she pressed a finger to her pursed lips telling him to keep quiet. He gave a slight nod and they both exchanged smirks before Isabelle decided to excuse herself from the room.

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**A/N: So what did you think? I hope it was okay. Please review and let me know! I worked pretty hard on this and I try not to disappoint. Oh, and if you are new to my work and you like it, then please go read and review my newly finished story about Isabelle and Magnus called 'Unforgettable'.**

**Anyways, all you readers out there! Please please please review. I would love to know what you thought about this!**


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